The parent application identified above describes a communication network of nodes, each of the nodes including a node processor for relaying messages between others of the nodes. A plurality of links connect the nodes in multiple rings of nodes, the network being divided into plural zones wherein each node responds only to a master node in the local zone. Those nodes located at boundaries between adjacent zones are gateway nodes and respond to master nodes of the adjacent zones, each of the gateway nodes being connected to nodes in adjacent zones of the network. Each node processor performs a distributed protocol for terminating a message if the message or a reply to the message from another node has been previously received or seen at the node. The node processors of the master nodes perform polling by transmitting a message authorizing a particular one of the other nodes in the local zone to transmit a new message, while the node processors in other nodes transmit a new message only upon receipt of the polling message from the local master node authorizing the particular node to transmit a new message. The polling feature permits all control of communication in the network to be concentrated in the master nodes.
One feature which maximizes the reliability of such a communication network is that the transmitting and relaying of messages (including polling messages) by each node in the network is performed in accordance with a flooding method in which each node broadcasts the message it is transmitting or relaying to all adjacent nodes. As a result, a message transmitted on the network will be broadcast eventually to all nodes, but, because of the "antibody" algorithm performed by each node (in which the node kills any message it has previously transmitted), the message is repeated by each node only once. The method of the invention "floods" the network with the message and chooses the shortest path between the transmitting and receiving nodes because it chooses all paths. Reliability of such a network is maximum because the loss of a single link (or even several links in most cases) cannot prevent the message from eventually reaching its destination, because the message travels on all links.
The entire network is monitored by each master node periodically polling all nodes within its zone. In doing so, the master node transmits a polling message addressed to a certain node, to which only the addressed node responds with an answering message. Once it receives the answering message, the master node repeats the process for the next node until all nodes in the zone have been polled. Thus, the master node transmits a succession of polling messages to successive nodes and receives successive responses. In the parent application, both the polling message and the answering message are flooded throughout the entire zone of the network. The poll message may be a request for data or a command to perform some function at the node: the network operation is the same.
While the foregoing provides an orderly and highly reliable polling system, it consumes a great deal of time in polling all nodes, which slows down the routine process of data acquisition, the primary application for which the protocol of the present invention is intended. This is because each polling message must be relayed through intervening nodes to the message's destination node, the requested operation (data acquistion or control) must be performed, that node's response must be relayed back and then the foregoing process must be repeated with the next node to be polled. Thus, the polling time for a network increases rapidly as a function of the number of nodes, a significant disadvantage. For example, in a simple version of the type of network described in the parent application comprising (in this example) a ring of ten nodes, one of which is a master node, polling of all nodes by the master node would require a total of 50 successive message transmissions ("hops") between adjacent nodes. Nevertheless, it has seemed that the disadvantage of the time required to poll the network was justified by the inherent reliability.
Accordingly, it is an object of the invention to reduce the amount of time required to poll all nodes without concomitantly reducing the reliability of the network.